A NOx storage catalyst which traps NOx (nitrogen oxides) in exhaust gas is used in the exhaust emission control device of an internal combustion engine.
The NOx storage catalyst traps and stores NOx in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean. When the engine is run at a rich or stoichiometric air fuel ratio, the stored NOx is reduced by components such as HC and CO in the exhaust gas.
However, SOx (sulfur oxide) is trapped by this NOx storage catalyst, too, and is stored. Therefore, the ability to trap NOx of an NOx storage catalyst falls when the amount of stored SOx increases (SOx poisonings).
In this regard, the exhaust emission control device disclosed in Tokkai Hei 7-217474 published by the Japanese Patent office in 1995 discloses a method wherein the SOx amount stored in the NOx storage catalyst provided in an exhaust pipe is estimated, and when the estimated SOx storage amount exceeds a permitted value, the exhaust gas flowing into the NOx storage catalyst is lean or at the stoichiometric air fuel ratio and catalyst temperature is higher than a predetermined temperature, the air-fuel ratio of the exhaust gas flowing into the catalyst is temporarily made rich, and stored SOx is discharged.